1. Field of Invention
The invention relates to an apparatus and a method of achieving cathodic corrosion protection for a surface of a metallic, aluminum-containing substrate, particularly a substrate of aluminum or an aluminum-based alloy, which can be flushed by an electrolyte.
2. Discussion of Related Art
The service life of objects which are made of metallic materials is limited because all metals react with materials from their surroundings, with the exception of a few, expensive noble or precious metals, whose usability is limited. As a result, metals degrade, which leads to aging and to a complete loss of function of any object that may have been manufactured from them. Such degradation and aging, which on the basis of chemical reactions lead to the disintegration of the manufactured object, are processes considered to be types of "corrosion".
A frequently observed corrosion is the so-called electrochemical corrosion, which always occurs when two different metals, which are connected electrically with one another, are flushed by the same electrolyte; such a combination of two metals with an electrolyte represents a short-circuited galvanic element, where one of the metals functions as an anode and the other as a cathode. The anode is corroded by the electrochemical
In this process initiated by the additional electrolytes manner, the anode is attacked from the surface and dissolved, Under some circumstances, this electrochemical process can also be exploited for the purpose of protecting against corrosion.
For example, if a metal is found which functions as anode when dipped in the same electrolyte as a different metal which is to be protected against corrosion, the metal to be protected will be so protected when brought into electrical conducting connection with the metal functioning as the anode. This concept is used in practice for the protection of iron against corrosion by seawater, in which the iron is connected with a "sacrificial anode" of zinc.
If it is not possible to select such a material or if such a selection must be rejected for reasons of protecting the environment (heavy metal ions are formed by the process described!), the electrochemical process can also be forced. This is done by introducing a power source instead of a short circuit connection between the two electrodes in the electrolyte, one electrode being the substrate to be protected and the other electrode being a corrosion-resistant counterelectrode. The substrate to be protected is connected as the cathode and the corrosion process can usually, in fact, be stopped or at least retarded significantly by selecting a suitable electric voltage.
Electrochemical corrosion protection methods are difficult to handle, where the substrate to be protected consists of aluminum or an alloy based on aluminum. In principle, aluminum is relatively a base metal so that corrosion protection with "sacrificial anodes" is more difficult. A method concerning "cathodic corrosion protection", which uses external voltage source, is also not readily usable.
In an oxygen-containing environment, aluminum is soon coated with a relatively impervious layer of oxide. Admittedly, this layer is very stable physically. However, because of the amphoteric character of aluminum oxide, it is attacked not only by acids, but also by alkalines. This aluminum oxide layer is stable only in an electrolyte, the pH of which lies between about 4.5 and about 8.5. Electrolytes, which are too acidic or too basic, attack the metal. When the usual, cathodic method of protecting against corrosion is used for aluminum or for alloys based on aluminum, the danger of corrosion by alkaline media is, in fact, present. The electrolysis which is required for the cathodic protection against corrosion causes the cathode to be charged with the cations from the electrolyte, which have been reduced by the electrolysis- As a result, an alkaline liquid boundary layer, which under some circumstances can lead to the alkaline corrosion of the aluminum, is formed in the vicinity of the surface of the object which is to be protected.
If an object of an aluminum-containing material is to be protected cathodically against corrosion, the chemical conditions existing at the object must be monitored very carefully so as to exclude the occurrence of alkaline corrosion with certainty. A modification of the method of cathodic protection against corrosion, in which a certain degree of control is given at least for the electrical conditions, is proposed in DE-U 8900911. This publication relates to a cathodic protection of a steel pipe against corrosion. The steel pipe encases a high-tension power line, so that it can be transferred underground. The counterelectrode of the pipe is the ground of a connecting station of the pipe. The pipe is acted upon by a negative voltage relative to ground. In order to observe the relevant protection regulations, the magnitude of the voltage of the pipe against ground must be reliably limited. It may be proposed that this be brought about by a number of diodes, which are connected in antiparallel fashion. However, this is not suitable for monitoring the electrochemical conditions at the cathode because not only the cathode is charged by the electrolysis that takes place between anode and cathode; the cathode is charged with reduced cations from the electrolytes and the anode is charged with oxidized anions. Consequently, because of the electrolysis, the electrochemical potential of both electrodes with respect to the electrolyte is changed. The voltage existing between the electrodes accordingly is not a measure of the conditions at an electrode. Instead, it is a measure of the total changes at the two electrodes and, with that, cannot be called upon for a reliable evaluation of the conditions at a single electrode, be it cathode or anode.
It would be desirable to provide a cathodic corrosion protection method for a surface of a metallic substrate, particularly an aluminum-containing substrate, which faces an electrolyte. With such a method, the electrochemical conditions at the surface, which is to be protected, may be monitored precisely and controlled directly to avoid formation of excessive alkaline boundary layers.